1. Field of the Invention
This invention relates to brazing of metal parts and, in particular, to a homogeneous, ductile brazing material useful in brazing stainless steels and high nickel alloys.
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, corrosion resistant, leak-tight joint is formed.
The brazing alloys heretofore used with stainless steels, designated AWS BNi compositions, contain a substantial amount (about 3 to 11 weight percent) of metalloid elements such as boron, silicon and/or phosphorus. Consequently, such alloys are very brittle and are available, for the most part, as powder, powder-binder pastes, powder-binder tapes and bulky cast preforms. Powders are generally unsuitable for many brazing operations, such as dip brazing, and do not easily permit brazing of complex shapes. Although some powders are available as pastes employing organic binders, the binders form objectionable voids and residues during brazing.
Some brazing alloys are available in full form. Such materials are generally fabricated by (1) rapidly quenching a melt of the composition on a rotating quench wheel at a rate of at least about 10.sup.5 .degree. C./sec to form a glassy metal, (2) rolling and heat-treating the composition or (3) powder metallurgical techniques. Rolled foil is not sufficiently ductile to permit stamping of complex shapes therefrom. Powder metallurgical foil is not homogeneous and employs binders, which form objectionable voids and residues during brazing. Glassy metal foils are not easily made and have braze-metal strengths somewhat lower than that of the joint's base metal with the result that the braze-metal oftentimes becomes the weakest link in the brazed joint. Moreover, the high-temperature strength of such glassy metal foils is generally lower than that considered desirable for brazing of components used in turbines, heat exchangers, evaporators and the like.
Glassy metal alloy coating compositions have been disclosed in U.S. Pat. No. 3,871,836, issued Mar. 18, 1975 to Polk et al. These alloys include compositions having the formula M.sub.a X.sub.b, where M is at least one element selected from the group consisting of Ni, Fe, Co, Cr and V, X is at least one element selected from the group consisting of P, B, C, Si, Al, Sb, Sn, In, Ge and Be, a ranges from 65 atomic percent to 90 atomic percent, b ranges from 10 atomic percent to 35 atomic percent, and up to about 1/3 of M can be replaced by at least one element selected from the group consisting of molybdenum, manganese, titanium, tungsten and copper. No brazing compositions are disclosed therein, however.
There remains a need in the art for an amorphous metal foil having increased braze-metal and high-temperature strengths.